Merge two or more blender files with the same model but with different animations?
Create a blendfile and import all the .fbx. and save it
Open the master blend file. (the one where all animations are supposed to go).
Click on File -&gt; Append
Choose the blendfile you created earlier containing all the .fbx.
Choose the 'folder' Action - Inside are all animations that were imported. Hopefully they have useful names.
Press A to select all and Append.
Import the main model into your .Blend.
Open an Action Editor and select the action you want to use with your selected object.
Just for safety check the Fake user icon for each animation.

Now you can switch, combine, select or even export to Unity etc. between your actions.

Btw. an FBX can store more than 1 animation. Usually you wouldn't store the same mesh several times, but once with multiple animations appended, or one fbx for each animation, with the mesh in a separate file.